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CHAPTER 9. CYTOGENETICS OF PEARL MILLET

CHAPTER 9. CYTOGENETICS OF PEARL MILLET

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408



PREM P. JAUHAR

E. Identification of Translocated Chromosomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

F. Disjunction of Interchange Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

G. Interchange Heterozygosity and Plant Breeding . . . . . . . . . . . . . . . .



444

445



IX. B Chromosomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A. B Chromosomes as Indicators of the Origin of Pearl Millet. . . . . . . . . . . . . . . . . . . 447



B . Mode of Pollination.. . . . . . . . . . . . . . . . . . . . . . . .



. . . . . . . . . . 454



XI. Hybridization and Chromosome Relationships . . . . . . . . . . . . . . . . .

....................................

A. lntraspecific Hybrids

B. Interspecific Hybrids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C. Intergeneric Hybridization . . . . . . . . . . . . . . . . . . . . . . . . . . . .

XII. Conclusion .

................................

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .



458



472

473



I. INTRODUCTION

Pennisetum is one of the most important genera of the tribe Paniceae of the

grass family. Pearl millet [Pennisetum ryphoides (Burm.) Stapf et Hubb.] is the

most important constituent of this genus. It is a dual-purpose crop: its grain is

used for human consumption and its fodder serves as feed for cattle. In Asia and

Africa, however, it is grown primarily as a grain crop on an estimated 60 million

acres of relatively poor land. It has remarkable ability to grow in areas of low

rainfall. In sub-Saharan Africa harvests of pearl millet are obtained with as little

as 250 mm of annual rainfall (Brunken, 1977). Its grain is traditionally considered to be nutritious and is put to a variety of uses. As poor man’s bread, it

sustains a large proportion of the populace of Africa and Asia. It also contributes

to the economy of countries like the United States, where it is grown as a forage

crop on an estimated 1 million acres. Pearl millet is also grown as a forage crop

in the tropical and warm-temperate regions of Australia and several other countries.

Pearl millet originated in West Africa. Selection exercised by the early cultivators under a myriad of cultural contexts led to the development of several

morphologically diverse forms. Its protogynous nature facilitated the introgression of characters from other wild and cultivated annual species of the section

Penicillaria. It is now widely cultivated in different parts of the world. In terms of

annual production, pearl millet is the sixth most important cereal crop in the

world, following wheat, rice, maize, barley, and sorghum. Among the millets it

is second only to sorghum. In India, it is the fourth most important cereal after

rice, wheat, and sorghum.



CYTOGENETICS OF PEARL MILLET



409



Pearl millet is a favorable organism for genetic research. Its chromosome

number, 2n = 14, was determined more than 50 years ago by Rau (1929).

Several favorable features of the chromosome complement, e.g., small number

and large size of chromosomes with one distinctive pair of nucleolar organizers,

make pearl millet a suitable organism for cytogenetic studies. Moreover, its

protogynous flowers and outbreeding system make it ideal for interspecific hybridization and for breeding work. It is indeed ideally suited for heterosis breeding. Although pearl millet has a remarkable ability to grow on soils of marginal

fertility, it responds very well to proper fertilization, which helps in realizing the

high yield potential of its hybrids. The hybrids’ greater N use efficiency (biomass

production per unit of N in the plant) is probably attributable to the highly

efficient (C,) photosynthetic pathway of this crop.

Although pearl millet has great agricultural importance, is a very favorable

organism for cytogenetic studies and breeding work, and has a low chromosome

number that was also determined at about the same time as those of most other

crops, the information available on its genetics and cytogenetics is much less

than that known for other important crops. There are several reasons why this

crop has been largely overlooked as a genetic and cytogenetic tool:



1. It has long been considered to be a crop of secondary importance and, thus,

could not compete for research funding with other crops like wheat and corn.

2 . It has a restricted area of use, being a food for the poor only, although it is

also an excellent fodder crop.

3. Its potential as a research tool was not appreciated until recently.

4. The existence of long-standing nomenclatural controversies (in the postLinnaean period from 1753 to 1759, pearl millet has been treated as a member of

at least six different genera, viz., Panicum, Holcus, Alopecurus, Cenchrus,

Penicillaria, and Pennisetum, and has been given different botanical names; see

Jauhar, 1981a) could also have had an adverse impact on research.

Studies on chromosome pairing in interspecific hybrids-with pearl millet as

one of the parents-have contributed to our understanding of phylogenetic relationships between different P ennisetum species and pearl millet. In these studies,

the large size of the pearl millet chromosomes has been helpful in ascertaining

chromosome relationships. Because of its low chromosome number, pearl millet

also offers a particularly favorable material for aneuploid analyses, which should

be helpful in the elucidation of its cytogenetic architecture. Primary trisomics

constitute a valuable tool for locating genes on different chromosomes and for

assigning them to linkage groups. Although considerable progress has been made

in developing a set of primary trisomics in pearl millet, the establishment of

linkage groups awaits completion. A good deal of information is available on

certain other cytogenetic aspects, e.g., polyploidy, interchange heterozygosity ,

haploidy, and B chromosomes. All these studies should contribute to the improvement programs of pearl millet.



410



PREM P. JAUHAR



The purpose of this article is to summarize and integrate the available information on different aspects of pearl millet cytogenetics. It is hoped that this article

will provide useful information to cytogeneticists and breeders engaged in the

improvement of pearl millet and other forage species of Penniseturn. This article

may also be of interest to a spectrum of other workers engaged in basic research.



II. KARYOTYPIC ANALYSIS

Karyotypic analysis includes the study of the number, size, and morphology of

chromosomes. Total length and arm ratios of chymosomes are helpful in systematic and phylogenetic investigations. Levitskii (193 1) and Avdulov (1 93 1)

pioneered the use of cytological features as aids in establishing taxonomic and

phylogenetic relationships among species and genera. Although basic number,

size, and morphology of the chromosomes can indeed be useful in taxonomic

classification (Hunter, 1934; Constance, 1957), these parameters should be subsidiary to morphological characters in any taxonomic treatment (Pilger, 1954).

Modern cytological techniques, e.g., the banding of chromosomes with Giemsa

(Vosa and Marchi, 1972; Vosa, 1973, 1975), and staining heterochromatic patterns with fluorochromes like quinacrine mustard (Vosa, 1970) can provide information of phylogenetic value.

The occurrence of cytotypes or chromosomal races (intraspecific polyploid

series) is a characteristic feature of the perennial species of Penniseturn. However, no such cytotypes exist in the annual cultivated or wild pearl millets, which

all have 2 n = 14 chromosomes (Table I); in fact, all these taxa belong to the

species P . ryphoides. There is a report of 2 n = 36 chromosomes for a Nigerian

collection of “ P . violaceurn (Lam.) L . Rich.” (Olorode, 1975), but this could be

an incorrect identification. Since the material classified as P . violaceurn forms

fully fertile hybrids with pearl millet (2n = 14), the former must have 2 n = 14

chromosomes (see Section XI,A).

KARYOMORPHOLOGY



Chromosomes are generally measured at somatic metaphase after pretreatments that condense and spread them. The main drawback inherent in these

studies is that the magnitude of error in the measurements of condensed chromosomes is high. Therefore relatively small size differences among chromosomes

of a species, of infraspecific categories, or of different species cannot be resolved

accurately. However, karyomorphological studies can be done more precisely on

pachytene chromosomes in taxa with low chromosome numbers, e.g., P .



41 1



CYTOGENETICS OF PEARL MILLET



TABLE I

Chromosome Numbers of Different Taxa in the Section Penicillaria of Genus Penniserurn

Taxa

Cultivated pearl millet

P. typhoides (Burm.) Stapf et Hubb.

[Syn. P. tvphoideutn Rich.

P. spicarurn (L.) Koern.

P. glaucum (L.) R. Br.

P. amerironum ( L . ) K. Schum.]

Annual relatives of pearl millet"

P. alhicauda Stapf et Hubb.

P. rmcvhchaele Stapf et Hubb.

P. rinereum Stapf et Hubb.

P. dalzielii Stapf et Hubb.

P. echinurus (K. Schum.) Stapf et Hubb.

P. fullax (Fig. & De Not.) Stapf et Hubb.

P. gambiense Stapf et Hubb.

P. leanis Stapf et Hubb.

P. tnuiwa Stapf et Hubb.

P. nigrilarurn Schlecht.

P. mollissimum Hochst.

P. perrottetii (Klotzch ex A.Br.)K. Schum

P. pyrnostachyum (Steud.) Stapf et Hubb.

P. pynostachvunr var. gambia

P. versicolor Schrad.

P. violareum (Lam.) L. Rich

Perennial relative of pearl millet

P. purpureutn Schum.



References



2n



14



14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

I4?

28



56



Rau (1929)



Thevenin (1952)

Krishnaswamy (1951)

Krishnaswamy (1951)

Krishnaswamy (1951);Thevenin (1952)

Krishnaswamy (1951)

Thevenin (1952)

Thevenin (1952)



Thevenin (1952)

Krishnaswamy (1951)

Bilquez and Lecomte (1969)

Mehra et ul. (1968)

Burton (1942); Nishiyama and Kondo

( 1942)

Krishnaswamy and Raman (1948);

Gadella and Kliphuis (1964)



"These and other annual, cultivated, or wild relatives of pearl millet have 2n = 14 chromosomes.

They are not reproductively isolated from the cultivated species-P. fyphoides-and in fact do not

deserve specific ranks.



ramosum ( 2 n = 10) and P . typhoides ( 2 n = 14). For critical comparisons, the

DNA content of chromosomes can also be measured.

The genus Pennisetum is a heterogeneous assemblage of species with chromosome numbers ranging from 2 n = 10 to 2 n = 7 2 , being multiples of 5, 7, 8, and

9. Their chromosome morphology is also very diverse, with tremendous size

differences; a noteworthy feature is that the species with lower numbers have the

larger sizes. Thus, pearl millet ( P . ryphoides) has only 2 n = 14, but relatively

very large chromosomes. Avdulov (1931) noted that pearl millet had 14 large

chromosomes, larger than those of any other member of the tribe Paniceae.



412



PREM P. JAUHAR



However, I think that the annual (or rarely biennial) species P . ramosum (2n =

10) has the largest chromosomes in the genus Pennisetum and probably in the

entire tribe Paniceae. The chromosomes of P . ramosum are approximately 5%

larger than those of P . typhoides. Thus, in the genus Pennisetum, the species

with the lowest chromosome number (2n = 10) has the largest chromosomes.

In contrast, the species with higher chromosome numbers (e.g., P . orientale,

2n = 18, 36, 54) have strikingly smaller chromosomes than those of P .

ramosum or P . typhoides. The trend of species with low chromosome numbers to

have much larger chromosomes is evident in several other plant groups. In

Sorghum, for example, the average lengths of chromosomes of S. versicolor (2n

= lo), S . vulgare (2n = 20), and S . halepense (2n = 40) were 4.86, 2.24, and

1.98 p m , respectively (Karper and Chisholm, 1936).

1 . P . typhoides (2n = 1 4 )



Rau (1929) determined from root tips the chromosome number of pearl millet

as 2n = 14. Moreover, he mentioned that “the chromosomes are very large” and

that the homologous pairs could be easily distinguished. Avdulov (1931) studied

the chromosomes of pearl millet, which was at that time classified as Penicillaria

spicata Willd. His drawing shows 14 chromosomes with median to submedian

centromeres, the shortest chromosome being the satellited one. It is interesting to

note that as early as 1931 when cytological techniques were not perfected,

Avdulov noticed one pair of satellited chromosomes; this observation has been

confirmed by numerous workers. The small nucleolar bivalent is clearly observed to be associated with the nucleolus (Fig. 1 ) .

Pantulu (1 958) examined the chromosomes at pachytene and grouped them

into four classes on the basis of relative length and position of centromere: ( 1 )

two large pairs (chromosomes 1 and 2) with median centromeres; ( 2 ) two somewhat shorter pairs (chromosomes 3 and 4) with median to submedian centromeres; (3) two medium-sized pairs (chromosomes 5 and 6) with submedian

centromeres; and (4)the shortest pair (chromosome 7) with the nucleolus organizer. Later, essentially similar results were obtained on the analysis of

karyotype at pachytene (Venkateswarlu and Pantulu, 1968; and Lobana and Gill,

1973), at pollen mitosis (Krishnaswamy and Raman, 1953a), and at somatic

metaphase (Burton and Powell, 1968). Virmani and Gill (1972) and Tyagi

(1975a) karyotyped the somatic chromosomes and classified them as follows:

chromosomes 1 , 2, 3, and 5 as metacentric; chromosomes 4 and 6 as submetacentric; and chromosome 7 as subterminal.

Thus, there are minor disagreements among different workers as to the position of centromere. Looking at a condensed chromosome at somatic metaphase,

it is not unexpected that one worker locates the centromere as median, whereas

another classifies it as submedian. The same workers, looking at pachytene and

somatic chromosomes, can also arrive at different conclusions. For example,



CYTOGENETICS OF PEARL MILLET



413



FIG. 1. Diakinesis in pearl millet ( 2 n = 14) showing one nucleolar bivalent; note small rod

associated with the nucleolus. Also note chiasma terminahation. [ X 12601



Virmani and Gill (1972) studied somatic chromosomes and classified chromosome 1 as metacentric; whereas, based on pachytene analysis, Lobana and Gill

(1973) considered it to be submetacentric.

There is no doubt that pearl millet has a fairly symmetrical karyotype. It is

certainly not very easy to identify all of the seven chromosomes by the techniques

currently used; therefore, Giemsa banding (see Vosa, 1973, 1975; Zelleret al.,

1977; Filion and Blakey, 1979) of somatic prometaphase chromosomes must be

tried to identify individual members of the complement. It has mostly metacentric or submetacentric chromosomes, the longest being approximately 1.5

times the shortest; both these features are indices of symmetry of karyotype.

Under Stebbins’ (1958) classification of types of asymmetry, pearl millet will fit

best in the class la, i.e., the most symmetrical of the 12 karyotypes described.

The shortest chromosome pair is somewhat subterminal with the satellite on its

short arm. It can be identified in somatic plates as well as at pachytene and

diakinesis, where, as a small bivalent, it is associated with the nucleolus (Fig. 1).

Chromosomes of some diploid taxa of the section Penicillaria, which are

annual relatives of pearl millet, have been observed. The materials classified as

Pennisetum cinereum, P. echinurus, P . gumbiense, P. leonis, and P. pycnosruchyum had 2n = 14 chromosomes, as in cultivated pearl millet (Krishnaswamy, 1951). Veyret (1957) found that P. ancylochaete, P . gambiense, P .

maiwa, and P. nigritarum had 2n = 14 chromosomes, and their chromosome

morphologies were similar to one another and also to that of cultivated pearl

millet. Genetic studies by Bilquez and Lecomte (1969) and Brunken (1977)



414



PREM P. JAUHAR



have shown that P . violuceum and P . fullux-two of the important wild, annual

relatives of pearl millet-are not reproductively isolated from it; their hybrids

with pearl millet were highly fertile. Although these workers have not mentioned

the chromosome number of these wild taxa, they obviously have 2n = 14

chromosomes in order to form fertile hybrids with pearl millet.



2. P . purpureum (2n



= 4x =



28)



Napier grass, an allotetraploid and a relative of pearl millet, has a somewhat

asymmetrical karyotype consisting of chromosomes with median, submedian,

and subterminal centromeres. On the basis of pachytene studies, Pantulu and

Venkateswarlu (1968) reported that the longest chromosome of the complement

(chromosome 1) was 2.7 times the length of the shortest (chromosome 14), thus

making the karyotype asymmetrical. Based on these observations, the karyotype

of P . purpureum will fall in the category 2b in Stebbins’ (1958) classification of

types of asymmetry.

Pantulu and Venkateswarlu (1968) reported that chromosomes 1 and 14 have

nucleolus organizers. The largest chromosome of the complement (chromosome

1) is certainly satellited, as evidenced by the association of the largest bivalent

with the nucleolus (Fig. 2 ) . The other nucleolar bivalent is one of the smallest, if

not the smallest, in the complement. If chromosome 14 is indeed satellited, then



FIG.2. Diakinesis in napier grass ( 2 n = 4x = 28) showing 14 bivalents (9 rings and 5 rods) with

terminalized chiasmata. Note one large bivalent (the largest in the complement) and one relatively

small bivalent associated with the nucleolus. Also note an additional small nucleolus (marked with

arrow). [ x 12601



CYTOGENETICS OF PEARL MILLET



415



P. purpureum shares an important karyotypic feature with P. typhoides, i.e., the

shortest chromosomes of both the species are satellited. Moreover, during meiotic prophase both typhoides and purpureurn show rapid terminalization of chiasmata (see Section 111). They also seem to have similar patterns of centromeric

heterochromatin. Thus, P. typhoides and P. purpureum seem to share some

important karyological features of phyletic value.



Ill. MEIOSIS

All penicillarias fall into the x = 7 group (see Table I). They have conspicuously penicillate anther tips (see Fig. 14a,b). Of these, only one species ( P .

purpureum) is a perennial tetraploid. All other taxa are annual and diploid with

2n = 2x = 14 chromosomes. The annual, semiwild taxa are not reproductively

isolated from the cultivated pearl millet and must be considered as infraspecific

categories within P . typhoides. They have regular meiosis with 7,,, as in P.

typhoides.

A . P . ryphoides ( B u R M . )STAPFET HUBB.( 2 n = 2x = 14)



Rau (1929) determined the chromosome number of pearl millet as 2 n = 14.

Rangaswamy (1935) studied meiosis and found at diakinesis mostly seven ringshaped bivalents having two terminalized chiasmata each.

In different populations of pearl millet, mostly ring bivalents with two chiasmata each are observed at diakinesis, but the nucleolar bivalent is generally a

small rod with one chiasma (Figs. 1 and 3b). The rapid terminalization of

chiasmata seems to be a characteristic feature, so that at diakinesis the bivalents

generally appear loose and dissociated (Figs. 1 and 3b,c). At metaphase, both

ring and rod bivalents are observed. In some cultivated varieties in India, the

mean chiasma frequency at metaphase was found to be 12.10 per cell and 0.86

per paired chromosome; this means that ring bivalents are preponderant (see Fig.

3d).

Some populations of pearl millet show secondary associations of bivalents.

Two groups of two bivalents each were clearly observed (Fig. 3c) in some cells.

Although the phyletic significance of secondary associations in diploid species

remains controversial, such associations cannot be entirely meaningless. In

hexaploid wheat, such associations are known to take place between genetically

and evolutionarily related chromosomes (Riley, 1960; Kempanna and Riley,

1964). In pearl millet, the secondarily associated bivalents look very similar to

each other, although their genetic and phyletic relatedness cannot be determined.

In the haploid complement when their homologous partners are missing, the

chromosomes involved in these secondary associations probably form bivalents.



416



PREM P. JAUHAR



FIG. 3. Meiotic stages in pearl millet. (a) Late diplotene showing 7 bivalents (711).(b) Diakinesis

with 711with teminalized chiasmata. Note 6 ring bivalents and the small, nucleolar rod bivalent. (c)

Diakinesis with 711.Note the secondary associations of two pairs of bivalents; the associated bivalents

look similar in size and shape. (d) Metaphase 1 with 611.[(a, d) X ca. 2050; (b, c) x ca. 21501



CYTOGENETICS OF PEARL MILLET



417



It is interesting to note that two bivalents have been reported in haploids studied

by different workers (see Section V,B, Table 11; Fig. 6c). These observations

lend favor to the suggestion that the complement of typhoides has been derived

from a basic set of x = 5 chromosomes (see Jauhar, 1968, 1970b; Sections V,B

and XI,B,l,c).

B . P . purpureum SCHUMACH.

( 2 n = 28, 56)



Elephant or napier grass ( P . purpureum) is a perennial relative of pearl millet

and is native to Africa. Burton (1942) and Nishiyama and Kondo (1942) determined its somatic chromosome number as 2n = 28, which is tetraploid based on

x = 7. It shows diploid-like meiosis, 14,, being regularly formed at diplotene,

diakinesis, and metaphase (Fig. 4a-c). No multivalents or univalents are generally formed. The occasional occurrence of a quadrivalent (Olorode, 1974) can be

attributed to a floating interchange in certain populations.

At diakinesis, there is a rapid terminalization of chiasmata (Figs. 2 and 4b)-a

feature also characteristic of pearl millet. Two bivalents are generally associated with the nucleolus (Fig. 4a,b). One of the nucleolar bivalents is the

largest in the complement, whereas the other is a small one (see also Section

11,2). Occasionally, additional nucleolar material is organized (see Fig. 2). At

metaphase, there are noticeable size differences among bivalents; the smaller

ones are generally rod-shaped with one chiasma each, whereas the majority of

the large ones are ring-shaped with mostly two chiasmata each (Fig. 4c).

Chiasma frequency per cell and per paired chromosome was found to be 18.9

and 0.68, respectively, in some collections.

Several factors speak for the allotetraploid nature of elephant grass: ( I ) its 2n

= 28 chromosome number; (2) the regular bivalent formation and high pollen

fertility; and (3) the noticeable size differences among bivalents. This is further

borne out by studies on chromosome pairing in its hybrids with pearl millet (see

Section XI,B,2,d). This allotetraploid can be genomically represented as

A'A'BB. The A' genome is homoeologous with the A genome of pearl millet

(which is genomically AA). The large bivalents observed at metaphase in P .

purpureum are evidently formed by the A'A' genome, whereas the small ones

belong to the BB genome, the donor of which is not yet known.



IV. ABNORMAL MEIOSIS AND ITS GENETICS

The nature of events that lead to synapsis and crossing over during the meiotic

prophase remains one of the most intriguing problems in cytogenetics today. It is



418



PREM P. JAUHAR



FIG. 4. Meiotic stages in napier grass ( 2 n = 4x = 28). (a) Early diakinesis with 14 bivalents

(l4,,). Note that two bivalents (one the largest in the complement and one much smaller) are

associated with the nucleolus. (b) Diakinesis with 14,: Due to terminalization of chiasmata, most

bivalents appear to be loose and dissociated. Note that 2,, are associated with the nucleolus; the large

bivalent-largest in the complement-is lying on the nucleolus. (c) Metaphase I with 14,,; I,, is

separated. [ x 14801



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